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In the present tense we use ‘can’ to talk about a general ability – e.g. I can swim – and also for ability at a particular moment – e.g. I can see you! The rules for talking about past abilities are different.
在現在時態里，我們用can去談論一般的能力，例如I can swim。還有形容在一個特定的時間的一種能力，例如I can see you！但是，關于談論過去的能力是不同的。
General ability 一般的能力
We use could to talk about general abilities in the past.
我們用could談論過去一般的能力。
• He could speak fluent French when he was 5.
• I could read before I started school.
Ability on one occasion 某個場合的能力
However, when we talk about ability on one particular occasion, we can’t use could但是，當我們談論某個特定場合下的能力，我們不能用could。
• The burglar was able to get in through the bathroom window.
• The burglar managed to get in through the bathroom window.
We have to use was able to or managed to. We can’t use could.
我們必須用was able to 或者managed to，我們不能用could。
• The burglar could get in through the bathroom window.
Ability on one occasion – negative 某個場合的能力—否定式
When we talk about a particular occasion when something wasn’t possible, we can use wasn’t/weren’t able to, didn’t manage to, or couldn’t
當我們談論某個特定場合不能做到某事的能力時，我們用wasn’t/weren’t able to, didn’t manage to, 或者是couldn’t
• The burglar wasn’t able to get in through the window.
• The burglar didn’t manage to get in through the window.
• The burglar couldn’t get in through the window.
Hypothetical past ability 假設的過去的能力
Sometimes things were possible in the past but didn’t happen.
有時候，我們要表達在過去可能會發生但是沒有發生的事情。
• I could have gone to university but I decided to get a job.
• I would have been able to win the race but I fell over.
We use could have (+ past participle) or would have been able to to talk about these hypothetical events. They can be used in the positive and the negative.
我們用could have (+ 過去分詞) or would have been able to 來談論這些假設會發生的事件。他們可以是肯定式，也可以是否定式。
• I couldn’t have done it without your help.
• I wouldn’t have been able to afford it even if it had been for sale.
（來源：英國使館文化教育處）